1. Field of the Invention
The present invention relates to air moving apparatuses and, more particularly, is directed to a device for reducing the distortion of air entering the inlet of a fan and the noise created thereby.
2. Description of the Invention Background
Over the years, a variety of devices have been developed for moving air and other gases. For example, various types of fans have been created for moving air for heating, ventilating and cooling purposes in residential and industrial structures alike. Virtually all refrigerators, freezers and air conditioners are equipped with a fan for moving air across their heat-exchanger coils. Fans are also frequently used in industrial applications for moving process air and contaminated air through filtration and pollution control systems. Electronic equipment may require cooling fans to prevent "hot spots" from developing within the equipment which could damage sensitive electrical components. Machines used to dry raw and processed materials use fans for circulating heated air to the product and for carrying moisture away from the materials. Air support structures require fans to inflate them and maintain their supporting pressure.
Fans are generally classified by the nature of the airflow through their impellers. Axial flow, radial flow (centrifugal), mixed flow and cross flow are types of fan impellers commonly employed. Perhaps the two types of fans that are most commonly employed are centrifugal fans and axial fans. The construction of a centrifugal fan and an axial fan are fundamentally different. The impeller of a centrifugal fan usually includes a front rim that has a centralized opening therein and a backplate that is attached in spaced-apart parallel relation to the rim by a series of radial blades. The impeller assembly is rotatably supported within a housing which has an inlet that corresponds with the opening in the impeller rim. As the impeller is rotated within the housing, air is drawn in through the inlet and into the center of the impeller. The centrifugal force developed by the impeller causes the air to be discharged radially out of the impeller and through an outlet formed in the housing; hence the name "centrifugal fan".
An axial fan is typically equipped with a "propeller-type" impeller that is rotatably supported within an air passage opening. For example, an axial fan may be mounted in a wheel or rim that is attached within an opening in a housing. As the impeller is rotated, air is drawn into or out of the housing depending upon the orientation of the impeller blades. Other axial fans are mounted within housings that can form portions of ductwork for carrying air for heating, ventilation and air conditioning purposes.
The selection of a particular size and type of fan for a particular application typically involves aerodynamic considerations, economic considerations and functional stability considerations. Axial fans are desirable air moving devices in most systems due to their relatively small sizes and high efficiencies. System design and fan applications, however, can be limited due to the axial fan's sensitivity to inlet air conditions. Axial fans often impart an air swirl at their inlets which can lead to an uneven velocity profile of inlet air immediately in front of the fan.
In addition, due to design considerations, the preferred configuration of many systems would require a change in air direction immediately in front of or at the rear of the air moving device. However, any obstruction or change in direction of airflow immediately in front of the fan can cause even more inlet air distortion which can result in a reduction in the fan's operating efficiency as well as impart cyclical stresses on the blades.
These undesirable conditions can also be caused when system components such as heat exchanging coils, sound attenuators, moisture eliminators, filters, etc. are located in close proximity to the fan inlet. It is common practice, therefore, to oversize such components to reduce the airflow distortion created thereby. Of course, such oversizing adds to equipment costs, operating costs and maintenance costs. Distortion of inlet air can also be caused by directing high velocity return air into a mixing device located in close proximity to the fan inlet. Existing building structure and design requirements also sometimes dictate that structural components (i.e., beams, joists, pipes, walls, etc.) pass through the fan inlet stream which can result in further airflow distortion.
In the past, the above-mentioned conditions were somewhat alleviated through the use of an "inlet leveling screen." An inlet leveling screen typically comprises a flat plate that has a plurality of perforations therethrough that comprise approximately fifty percent of the plate area. While such a device causes the inlet air to be more evenly distributed across the screen and thus reduces the distortion of the air as it enters the fan, it creates added airflow resistance which places a greater load on the fan motor often requiring larger, more expensive motors to be used thereby adding to equipment and operating costs. In this device, the airflow remains in an axial direction and thus objects such as heat exchanger coils, noise attenuators, filters, etc. that are placed immediately in front of the screen can limit its effectiveness.
The effectiveness of prior air inlet level screens is also limited by the screen's surface area. Thus, traditional inlet leveling screens are typically constructed with a "round-to-square" transition member attached to the inlet end of the fan housing which enables the screen area to be somewhat maximized. Such arrangements, however, are usually very large and cumbersome which makes them expensive to build and difficult to install. Further, such devices usually cannot be used in applications where space is limited.
Other fan inlet devices have been developed and are disclosed in U.S. Pat. No. Re 31,258 to De Baun, U.S. Pat. No. 3,483,676 to Sargisson, U.S. Pat. No. 3,519,024 to Johnson et al., U.S. Pat. No. 3,871,844 to Calvin, Sr., U.S. Pat. No. 5,099,879 to Baird and U.S. Pat. No. 5,405,106 to Chintamani et al. Devices of the types disclosed above are typically expensive to produce and install. In addition, such devices often require the use of large motors for operating the fan. Moreover, those prior devices often occupy large amounts of building space which might otherwise be used for other purposes.
Other fan-related problems exist in air distribution systems for buildings and commercial structures. Such systems typically comprise discrete functional elements coupled together in series at a central location in a building. Such a system usually includes an input plenum for mixing outside and "return" air, filters, heat exchanging coils, a fan and noise attenuation apparatus for reducing the noise created by the airflow. Because such components typically occupy large amounts of building space when linearly-aligned, it often becomes necessary to arrange components in non-linear orientations. For example, structure design considerations sometimes require that inlet ducts for fans be orientated at right angles relative to the fan inlet. In addition, because relatively high airflow velocities are required to service large buildings, sound attenuating apparatuses must be employed. However, prior sound attenuating apparatuses are typically large and expensive and difficult to manufacture and install or they are relatively small devices which undesirably restrict airflow which increases airflow distortion.
Thus, there is a need for a device for reducing distortion of airstream entering the inlets of fans without greatly adding to the airflow resistance.
There is a further need for an airflow inlet device that is small and relatively easy to install and inexpensive to produce.
There is yet another need for a fan inlet device that can be used in close proximity to coils, filters, etc. and effectively minimize the airflow distortion entering the fan's inlet.
There is still another need for a device that can reduce the distortion of an airstream in a system to such a degree such to enable axial fans to be used in applications where their uses would have otherwise been prohibited.
Another need exists for a compact air handling system that can provide airflows similar to airflows typically achieved by prior systems that occupy large spaces.
Yet another need exists for an air handling system with improved silencing characteristics.